The present invention relates to a spatial light modulating device which performs optical information processing using a microchannel spatial light modulator.
A microchannel spatial light modulator (MSLM) has been known as an electron tube that performs incoherent to coherent conversion of light. A spatial light modulating device that uses a microchannel spatial light modulator is described in, for example, Japanese Patent Application Unexamined Publication Nos. 64742/1983 and 43045/1987.
The principle of optical information processing with a spatial light modulating device is shown in FIG. 4. A microchannel spatial light modulator generally indicated as 3 comprises a photoelectric conversion means including a photocathode 4, a microchannel plate (MCP) 5 for electron multiplication that is disposed at the electron-emitting side of the photocathode 4, a mesh electrode 6 used to collect secondary electrons emitted from the surface of an electro-optic crystal plate 7, and the electro-optic crystal plate 7 typically made of LiNbO.sub.3 (lithium niobate). These components are sealed in a vacuum envelope.
When an incoherent input optical image is incident on the photocathode 4, it emits photoelectrons to create an electron image. This electron image is amplified with MCP 5 and passes through the mesh electrode 6 to fall on the electro-optic crystal plate 7 where an electric charge image is formed. Since the electric field traversing the crystal plate 7 is dependent on the quantity of electric charge, the refractive index distribution of the crystal will change by Pockels effect. When the electro-optic crystal plate 7 is illuminated with a laser beam (linearly polarized laser beam) through a half mirror 8, a phase change is induced in the light reflected from the surface of the electro-optic crystal plate 7 (the surface on the side closer to the photocathode 4) according to the quantity of electric charge. Thus, by allowing the reflected beam to pass through a polarizing plate 31, a coherent output optical image is obtained as modulated with the incoherent input optical image.
The microchannel spatial light modulator that employs the photocathode 4 as an electron source is generally referred to as an "optically addressable type" and features several capabilities for processing input information, such as threshold operations, logic operations and contour extraction.
The optically addressable type of microchannel spatial light modulator, however, has the problem of difficulty in entering a video input because of the need to create an input optical image on the photocathode 4. Another disadvantage is the complexity of systems such as a lens system. It has therefore been proposed that the photocathode be replaced by an electron gun to perform electric addressing. Even in this approach, an electron lens system and an electromagnetic coil are needed to focus and deflect electron beams. As a further problem, some interface with a computer is required if one wants to write in two-dimensional information obtained by calculations on the computer.
The forementioned Publication No. 64742/1983 proposes a technique that has been developed to solve these problems of the prior art by using a matrix array of light-emitting elements. However, on account of the non-uniformity in the luminous efficiency of the light-emitting elements, the resultant output image will not have a uniform distribution of intensity. In order to provide uniformity in the intensity of output image, the writing time needs to be extended but then high-speed processing becomes impossible. Another problem is the difficulty in improving resolution owing to the size of the light-emitting elements. Improved resolution cannot be attained without unduly increasing the size of the matrix of light-emitting elements as well as the size of the lens system.